Penarth branch line loop

When it became clear that a branch line at Penarth would not be able to handle an expected increase in trains, Amey Consulting designed a solution based on an innovative modelling exercise

KeolisAmey, a joint venture between Amey and transport operator Keolis, has undertaken an ambitious 15-year programme to transform the Wales & Borders rail network on behalf of Transport for Wales. This overarching contract covers a multitude of projects aimed at enhancing and upgrading the rail service, including the upgrade of the Core Valley Lines (CVL).

The CVL, part of the South Wales Metro, is undergoing a comprehensive programme of refurbishment and renewal to enable its ageing infrastructure to support new developments such as electrification and the introduction of a fleet of light rail vehicles (LRVs). These works generally involve multiple disciplines and require close collaboration to ensure the best outcomes. A good example of what this means in practice is the Penarth branch line project.

Scheduled services on the CVL are expected to increase significantly with the arrival of the new trains. However, Amey’s rail operations team recognised that the Penarth branch line – an offshoot from the main line at Cogan junction – would not be able to cope with the expanded capacity. The single-track line is only able to accommodate one train at a time, and is currently occupied for 12 minutes out of every 15. Delays force incoming trains to wait on the main line for the branch to become vacant. One late train can block up the entire network. This situation would only get worse with the expanded timetable.

Amey Consulting was tasked with developing a solution that would enable two trains to occupy the branch line at the same time. A multidisciplinary workshop involving the design, signalling, track and electrification and power (E&P) teams was held to investigate different options. This resulted in a proposal to add a short loop at the Penarth end of the branch with an additional platform, allowing a second train to enter the loop and disembark its passengers before the earlier train leaves the main platform.

However, matters were complicated by limited space at the Penarth end of the line, imposing significant constraints on the design. The Amey team undertook an innovative value engineering exercise to establish the shortest feasible length for the extra track. This involved modelling the timetable against different loop lengths and applying different train delay scenarios, resulting in an efficient track design which maximises value and reduces the environmental impact of the works.

Space constraints also posed difficulties for the signalling, in particular the provision of an overlap (minimum 75-metre length of track to accommodate train overruns) at the loop exit signal.

The easiest option – allowing for an overlap across the exit points and on to the existing rail line – was rejected because of the risk to efficient operation of the branch line. Moving the proposed location of the new platform and exit signal to contain the overlap within the loop, clear of the points, would oblige passengers to walk further to reach the exit. Ultimately the workshop agreed the best option was providing a run-off ahead of the loop exit signal, with a buffer stop at 75 metres.

The outcome of the project is an efficient and compliant design that solves the problem of the Penarth branch line pinch point, clearing the way for the influx of trains expected as part of the CVL expansion.